In an internal combustion engine, a crankshaft may be used to convert the reciprocating motion of the pistons into rotational motion. The crankshaft may include, offset from its rotational axis, a plurality of crankpins. Each crankpin is coupled through a bearing to a piston rod, which is linked to a corresponding piston. The crankshaft itself is supported by two or more main bearings, in which it rotates.
To prevent excessive vibration while rotating, the crankshaft must be accurately balanced. To this end, the crankshaft may include a series of counterweights that counterbalance the mass of each crankpin, associated piston, piston rod, and entrained lubricant. In a traditional approach, the crankshaft may be constructed with initially oversized counterweights. Then, in a subsequent balancing step, lightening bores may be drilled into the counterweights to remove mass and thereby bring the crankshaft into balance. In this approach, the lightening bores may be arranged virtually anywhere on the counterweight, with no adverse effect on the operation of the crankshaft.
The approach summarized above may prevent excessive vibration of a crankshaft perpendicular to its rotational axis, but does not address the torsional vibration that may occur about the rotational axis. More specifically, each piston rod transmits a torsional impulse to its attached crankpin during the power stroke of the associated piston. With each torsional impulse received, the crankshaft twists slightly about its rotational axis, then twists back after the power stroke. In this manner, periodic torsional impulses from each of the piston rods may drive a complex torsional vibration in the crankshaft. Depending on conditions, such vibration may coincide with an order of natural resonance frequency of the crankshaft. When this occurs, the vibration may increase in amplitude, such that the crankshaft is inelastically deformed, causing material failure.
Certain crankshaft components may be used to suppress torsional vibration and thereby protect the crankshaft from material failure—flywheels and torsional dampeners, as examples. Another approach is to install one or more so-called ‘pendulum absorbers’ on the crankshaft. A pendulum absorber is a mass non-rigidly linked to the crankshaft at a predetermined distance from the rotational axis of the crankshaft. When the crankshaft receives a torsional impulse at a crankpin, that impulse is partly absorbed in accelerating the mass of the pendulum absorber in the direction of the impulse. Likewise, when the crankshaft relaxes after the impulse, the relaxation is opposed by the inertia of the mass that was accelerated. In order to suppress natural vibrational modes of a crankshaft, a pendulum absorber may be ‘tuned’ by adjustment of its mass M and of the distance L between its center-of-mass and the rotational axis of the crankshaft.
In some crankshafts, pendulum absorbers may replace some or all of the traditional counterweights, providing balance as well as torsional-vibration absorption. With this configuration, however, it is not possible to use the traditional balancing approach—i.e., to oversize the pendulum absorber and then drill an appropriately sized lightening bore into it, to reduce the mass. This is because the mass M of the pendulum absorber affects the order to which it is tuned for a given rotational speed and cylinder firing order.
Accordingly, one embodiment of this disclosure provides an altogether different method of balancing a crankshaft. The method includes connecting a torsion-absorbing pendulum to a cheek of the crankshaft, which is coupled to a crankpin. The pendulum connected to the cheek has insufficient mass to balance the crankshaft. A lightening bore is then formed through a bisection plane of the cheek. In forming the lightening bore, enough material is removed from the cheek so that the mass of the pendulum becomes sufficient to balance the crankshaft. In this example method, the bisection plane includes both a rotation axis of the crankpin and a rotation axis of the crankshaft. By boring into the cheek instead of the pendulum, the desired torsional-vibration absorbing properties of the pendulum are unchanged by the balancing procedure. Moreover, the placement of the lightening bore within the bisection plane of the cheek preserves the structural integrity of the crankshaft.
The summary above is provided to introduce a selected part of this disclosure in simplified form, not to identify key or essential features. The claimed subject matter, defined by the claims, is limited neither to the content of this summary nor to implementations that address the problems or disadvantages noted herein.